St Mary Redcliffe Project 450 RIBA 2 Stage End Report

9 Mechanical Services Strategy
9.1 Heating
Insulation
Passive Design Measures
Reducing the U-values of external building elements will significantly reduce heat loss through
the building fabric. This in turn will mean that less energy is required to condition the building.
A U-value is a measure of heat loss. It is expressed in W/m²K and shows the amount of heat lost
in watts (W) per square metre of material (for example wall, roof, floor etc.) per degree of
temperature difference between indoor and outdoor temperatures in Kelvin. The lower the U-
value, the better the insulation provided by the material.
It will be very hard to improve the thermal performance of spaces within the church. The new build extensions however
give a perfect opportunity to beyond current building regulation standards and even potentially looking at passivhaus
methodology if zero carbon is on the agenda after the sustainability workshop. The below table shows the current
proposed U-values compared to building regulation min standards.
Building Fabric New thermal elements Part L2B Proposed new elements
External wall 0.28 W/m².K 0.18 W/m².K
Roof 0.18 W/m².K 0.16 W/m².K
Floor 0.22 W/m².K 0.16 W/m².K
Glazing 1.8 W/m².K 1.4 W/m².K
Glazing g-value 0.64 Variable
External Doors 1.8 W/m².K 1.4W/m².K
Airtightness
Increasing the air tightness of the building through careful detailing and accurate construction techniques will also reduce
the heating demand. Building airtightness can be defined as the inward or outward air leakage through unintentional
leakage points or areas in the building envelope. This air leakage is driven by differential pressures across the building
envelope due to the combined effects of stack, external wind and mechanical ventilation systems. It is measured in m³ or
air, per m² of envelope, per hour at 50 Pascals differential pressure between the inside and outside of the building.
Airtightness is crucial to improving the energy performance of buildings. In the UK, the temperature of the outside air is
nearly always lower than the temperature of the air inside the building, thus, any air leakage
from the inside to the outside of the building is likely to result in:
1. A significant reduction in the thermal resistance of the thermal insulation, due to air
leakage past the insulation (thermal bypassing), leading to increases in realised fabric U-
values.
2. An increase in the building’s ventilation and fabric heat losses, resulting in an increase in space heating
requirement.
Thermal mass
Thermal mass describes the ability of a
material to absorb, store and release
heat energy with the aim of
moderating the internal environment.
During the day in winter, exposed
thermal mass absorbs and stores heat.
As the temperature drops at night the
thermal mass radiates the heat stored
from the day. This cycle is repeated daily.
Thermal mass is not a crucial aspect of sustainable heating solutions and is most relevant to the control of summer
temperatures.
The existing parts of the building has a lot of exposed thermal mass due to the age of construction. This will help reduce
the cooling demands in areas with high internal gains and also allow the option of looking into a natural ventilation
strategy in certain areas.
Heat Generation Options
As the existing heating plant serving the church has been reported as functioning well, it is not
proposed to modify this to increase capacity for the new areas. The new development will instead
have its own dedicated plant. During stage 2, a number of heat generation options have been
reviewed. These are listed below:
1. Gas fired boilers
2. Air Source Heat Pumps (ASHP)
a. Air to Water heat pumps
b. Variable refrigerant flow (VRF)
3. District heating connection
4. Packaged air handling plant providing space heating
Gas Fired Boilers - A dedicated plant room would need to be provided to serve a new Low Temperature Hot Water (LTHW)
circuit. This system would connect well into a district heating network should the network be sufficiently expanded in line
with the construction programme. This option would also include all associated pressurization, pumping and controls
plant.
Building Air Permeability New thermal elements Part L2B Proposed new elements
Air-tightness standard 10 m³/h.m² @50Pa 3 m³/h.m² @50Pa
1563.R1 – Stage 2 report Page 14 of 29